Lithium ion batteries have high specific energy, good rechargeable performance and low usage loss, etc., so they are commonly used in consumer electronics and electric vehicles. At present, chemical batteries with high energy efficiency and high density generally use organic liquid electrolytes, which are volatile, flammable, and easily to cause liquid leakage. This kind of chemical battery needs to have multiple protection means, thus causing the battery system complex in structure and high in cost. Although gel polymer electrolytes have similar performances as the solid electrolyte such as high safety, and similar performances as the liquid electrolyte such as high conductivity and high rate capability, which solves the security problem of lithium ion battery to some extent. However, the gel polymer electrolyte also uses liquid organic solvent as a plasticizer, and thus the safety problem remains to be solved.
A solid inorganic electrolyte is also known as lithium super ionic conductor, which has high Li+ conductivity, large transport-number, low conductance activation energy, good high temperature resistance, so it is widely applied in large power lithium ion batteries having high specific energy. Replacing the organic liquid electrolyte with a lithium ion solid inorganic electrolyte, disadvantages such as a short circuit in the battery and liquid leakage may be avoided, and the usage safety of lithium ion batteries may be enhanced. Thus, researches on solid electrolytes remain hot in the lithium ion battery field.
The current research on inorganic solid electrolyte for lithium ion batteries focuses on: LISICON (Li2+2Zn1−xGeO4) solid electrolyte, NASICON (Na superionic conductor) solid electrolyte; solid electrolyte with a perovskite structure, crystalized lithium ion solid electrolyte with a garnet-like structure, glassy lithium ion solid electrolyte of oxide type, glassy lithium ion solid electrolyte of sulfide type, and glassy lithium ion solid electrolyte of a combination of oxide and sulfide. These solid electrolytes improve the security, and they can work at high temperatures. Especially, the NASICON compound is a lithium ion organic solid electrolyte which can conduct lithium ions at a high speed, so it is being widely used in developing solid electrolytes for completely solid-state secondary battery.
Currently, NASICON type electrolyte has been modified by ion doping and has obtained a higher room temperature ionic conductivity (for example, larger than 10−4s/cm). However, the NASICON type solid electrolyte has a high resistance at the grain boundary between solid particles, and poor compatibility with electrode materials, which greatly limits its application in completely solid-state lithium ion batteries.
Chinese patent Publication No. CN101894972A discloses a NASICON solid lithium ion electrolyte with a chemical formula Li1+2(X1+X2)+2y+2ZAlX1ZnyM2−(X1+X2)−ySiX2P3−(X1+X2)O12−Z, where M is Ti, Ge, or Zr, and 0.1≦X1≦0.5, 0.1≦X2≦0.5, 0.01≦y≦0.1, 0.5≦Z≦3.6. The NASICON solid lithium ion electrolyte has lithium ion conductivity larger than 10−4 S/cm, but it still has a relatively higher resistance at grain boundary between the particles and poor compatibility with electrode materials.